Device for generating x-rays having a heat absorbing member

ABSTRACT

The invention relates to a device for generating X-rays ( 41 ). The device comprises a source ( 5 ) for generating an electron beam ( 35 ), and a carrier ( 7 ) which is rotatable about an axis of rotation ( 15 ) and which is provided with a material ( 9 ) which generates the X-rays as a result of the incidence of the electron beam thereon. The device further comprises a heat absorbing member ( 45 ) which is arranged between the source and the carrier to catch electrons, which are scattered back from an impingement position ( 39 ) of the electron beam on the carrier, and to absorb a portion of the radiant heat generated by the carrier when heated during operation. The heat absorbing member is in thermal connection with a cooling system ( 51 ) of the device. According to the invention, the thermal connection between the heat absorbing member ( 45 ) and the cooling system ( 51 ) comprises a thermal barrier ( 57 ) which limits a rate of heat transfer (( ) occurring via the thermal connection per unit of temperature difference between the heat absorbing member and the cooling system. In a particular embodiment, said thermal barrier comprises an annular mounting member ( 57 ) having a limited dimension (hB), by means of which the heat absorbing member is mounted in the device. As a result of said thermal barrier, the heat absorbed by the heat absorbing member is gradually transferred to the cooling system, so that thermal peak loads on the cooling system and problems like boiling of the cooling liquid are avoided. In addition, relatively high temperatures of the heat absorbing member are allowed, so that the mass and volume of the heat absorbing member, which are necessary to provide the heat absorbing member with a sufficiently large heat absorbing capacity, are considerably reduced.

The invention relates to a device for generating X-rays, which devicecomprises a source for emitting electrons, a carrier which is rotatableabout an axis of rotation and which is provided with a material whichgenerates X-rays as a result of the incidence of electrons, a heatabsorbing member arranged between the source and the carrier, and acooling system which is in thermal connection with the heat absorbingmember, wherein during operation a rate of heat absorption by the heatabsorbing member is substantially larger than a rate of heat transfervia the thermal connection.

A device of the kind mentioned in the opening paragraph is known fromU.S. Pat. No. 6,215,852. The source, the carrier, and the heat absorbingmember are accommodated in a vacuum space of the device. The carrier isdisc-shaped and is rotatably journalled by means of a bearing. Duringoperation, an electron beam generated by the source passes through acentral cavity provided in the heat absorbing member and impinges uponthe X-ray generating material of the carrier in an impingement positionnear the circumference of the carrier. As a result, X-rays are generatedin said impingement position, which emanate through an X-ray exit windowprovided in a housing enclosing the vacuum space. The heat absorbingmember has the same electrical potential as the carrier and is arrangedbetween the source and the carrier to catch electrons, which arescattered back from the carrier, and to absorb radiant heat generated bythe carrier when heated during operation, as a result of which the heatabsorbing member is heated during operation. The cooling systemcomprises a channel for a cooling liquid, which is provided in acircumferential portion of the heat absorbing member in direct thermalcontact with the heat absorbing member. As a result, the thermalconnection between the heat absorbing member and the cooling system hasa relatively high thermal conductivity. The heat absorbing member ismade from copper and has a relatively large mass and volume, so that theheat absorbing member has a large heat absorbing capacity. Thus, whenthe device is temporarily in operation to generate X-rays of arelatively high energy level, a relatively large rate of heat absorptionby the heat absorbing member temporarily occurs, during which the heatabsorbing member undergoes a moderate temperature increase only. As aresult of said moderate temperature increase, the rate of heat transferfrom the heat absorbing member to the cooling system is limited, and theheat absorbed by the heat absorbing member is gradually transferred tothe cooling system during the time that the device generates X-rays andafterwards when the device is not in operation. As a result of saidgradual transfer of the heat from the heat absorbing member to thecooling system, thermal peak loads on the cooling system are prevented,so that cooling system problems, such as boiling of the cooling liquidor melting of thin-walled structures of the cooling system, areprevented.

A disadvantage of the known device is that the device has relativelylarge dimensions and a relatively large weight as a result of therelatively large mass and volume of the heat absorbing member.

It is an object of the invention to provide a device for generatingX-rays of the kind mentioned in the opening paragraph, which also hasthe advantage of a gradual transfer of heat from the heat absorbingmember to the cooling system, but in which the mass and volume of theheat absorbing member are significantly reduced.

To achieve this object, a device for generating X-rays according to theinvention is characterized in that the thermal connection between theheat absorbing member and the cooling system comprises a thermal barrierwhich limits the rate of heat transfer, occurring via the thermalconnection per unit of temperature difference between the heat absorbingmember and the cooling system, in a predetermined manner. In the deviceaccording to the invention, a gradual transfer of heat from the heatabsorbing member to the cooling system is not achieved by moderating themaximal temperature reached by the heat absorbing member during thegeneration of X-rays, as in the device known from U.S. Pat. No.6,215,852, but by limiting the rate of heat transfer which occurs viathe thermal connection per unit of temperature difference between theheat absorbing member and the cooling system, i.e. by limiting thethermal conductivity of the thermal connection. As a result, arelatively high maximal temperature of the heat absorbing member isallowed during the generation of X-rays, provided that the heatabsorbing member is made from a suitable material having a sufficientlyhigh melting temperature. As a result of the relatively high maximaltemperature allowed, only a relatively small mass and volume of the heatabsorbing member are required to enable the heat absorbing member toabsorb a total amount of heat comparable to the amount of heat absorbedby the heat absorbing member of the known device. Since the necessarythermal conductivity of the thermal connection is limited, less highdemands have to be made also upon the thermal conductivity of thematerial of the heat absorbing member, so that a range of suitablematerials for the heat absorbing member is not limited by demandsimposed on the thermal conductivity of the material.

A particular embodiment of a device according to the invention ischaracterized in that a cross-sectional heat transfer coefficientθ=φ/P_(max) of the thermal connection is smaller than 0,0005 K⁻¹,wherein (in kW/K) is the rate of cross-sectional heat transfer via thethermal connection per unit of difference between an average temperatureof the heat absorbing member and a temperature at a thermal boundarybetween the thermal connection and the cooling system, and whereinP_(max) (in kW) is a maximal output power of the source allowed duringcontinuous operation of the device. If said cross-sectional heattransfer ratio θ is smaller than 0.0005 K⁻¹, a relatively high maximaltemperature of the heat absorbing member is achieved during operation,so that the mass and volume of the heat absorbing member, which arenecessary to enable the heat absorbing member to absorb a sufficientlylarge amount of heat, are considerably reduced.

A particular embodiment of a device according to the invention ischaracterized in that the thermal barrier comprises a mounting member bymeans of which the heat absorbing member is mounted in the device, saidmounting member having a dimension, seen in a direction parallel to anelectron beam path of the source, which is substantially smaller than adimension of the heat absorbing member in said direction. In thisembodiment the mounting member, which is necessary to mount the heatabsorbing member in the device, also constitutes the necessary thermalbarrier or a part thereof, as a result of which the device has a simpleconstruction with a limited number of parts. Since said dimension of themounting member is relatively small, the mounting member has arelatively small cross-sectional area, as a result of which the rate ofheat transfer, occurring via the thermal barrier per unit of temperaturedifference between the heat absorbing member and the cooling system, iseffectively reduced. A predetermined limitation of said rate of heattransfer can be achieved by a suitable value of said cross-sectionalarea, i.e. by a suitable value of said dimension of the mounting member.

A further embodiment of a device according to the invention ischaracterized in that the heat absorbing member is substantiallyrotationally symmetrical relative to the electron beam path, and themounting member is annular and concentric relative to the electron beampath. In this further embodiment, the heat absorbing member is evenlywarmed up by the electrons scattered back from the carrier, and the heatabsorbed by the heat absorbing member is evenly transferred, seen in acircumferential direction of the annular mounting member, via themounting member to the cooling system. In this manner, the risk ofexcessive local temperatures of the heat absorbing member, the mountingmember, and the cooling system is considerably reduced.

A further embodiment of a device according to the invention ischaracterized in that the mounting member is made from a material havinga thermal conductivity which is lower than a thermal conductivity of amaterial from which the heat absorbing member is made. Since the thermalconductivity of the material of the mounting member is lower than thethermal conductivity of the material of the heat absorbing member, therate of heat transfer, occurring via the mounting member per unit oftemperature difference between the heat absorbing member and the coolingsystem, is effectively reduced.

A further embodiment of a device according to the invention ischaracterized in that the mounting member is made from stainless steel.Stainless steel is a very suitable material for the mounting member inview of its heat conducting properties, its thermal expansionproperties, and its mechanical properties.

A further embodiment of a device according to the invention ischaracterized in that the heat absorbing member has a first side facingthe carrier and a second side facing away from the carrier, the mountingmember being in thermal contact with the heat absorbing member near saidsecond side. Near the second side, during operation, the heat absorbingmember has a temperature which is lower than an average temperature ofthe heat absorbing member and lower than a temperature near the firstside. As a result, the rate of heat transfer from the heat absorbingmember to the cooling system via the mounting member is further reduced,so that the transfer of heat from the heat absorbing member to thecooling system takes place even more gradually.

A particular embodiment of a device according to the invention ischaracterized in that the thermal barrier comprises a vacuum gap whichis present between a radiant heat transferring surface of the heatabsorbing member and a radiant heat transferring surface of the coolingsystem. In this embodiment, the heat absorbing member is mounted in thedevice by means of, for example, a mounting member which is preferablymade from a thermally insulating material. Thus the transfer of heatfrom the heat absorbing member to the cooling system mainly takes placeby heat radiation via said vacuum gap, as a result of which the rate ofheat transfer, occurring via the thermal barrier per unit of temperaturedifference between the heat absorbing member and the cooling system, iseffectively reduced. A predetermined limitation of said rate of heattransfer can be achieved by suitable values of the areas of said radiantheat transferring surfaces of the heat absorbing member and of thecooling system and by a suitable value of the width of the gap.

A particular embodiment of a device according to the invention ischaracterized in that the heat absorbing member is made from molybdenum,tungsten, or graphite. Said materials have relatively high meltingtemperatures, so that relatively high temperatures of the heat absorbingmember are allowed, and so that the mass and volume of the heatabsorbing member, which are necessary for a sufficient rate of heatabsorption by the heat absorbing member, are considerably reduced.

A particular embodiment of a device according to the invention ischaracterized in that a side of the heat absorbing member facing thecarrier has an electron absorbing surface which is concave as seen froman impingement position of the electrons on the carrier. The electronsscattered back from the impingement position have an energy level whichdepends on an angle α at which the electrons are scattered back relativeto the path of the electron beam generated by the source. Said energylevel is approximately proportional to sin(2α), so that said energylevel increases from approximately 0 at α=0° to a maximal valueapproximately at α=45°. As a result of the fact that the electronabsorbing surface of the heat absorbing member is concave, the portionof the electron absorbing surface available to catch the electronsscattered back at a certain angle α also increases between α=0° andα=45°. As a result, a substantially uniform rate of heat absorption perunit of area of the electron absorbing surface is achieved, so that theheat absorbing member is substantially uniformly heated up by thescattered electrons and excessive local temperatures of the heatabsorbing member are avoided.

In the following, embodiments of a device for generating X-raysaccording to the invention will be described in detail with reference tothe Figures, in which

FIG. 1 schematically shows a longitudinal section of a first embodimentof a device for generating X-rays according to the invention,

FIG. 2 schematically shows a heat absorbing member of the firstembodiment of FIG. 1, and

FIG. 3 schematically shows a heat absorbing member of a secondembodiment of a device for generating X-rays according to the invention.

The first embodiment of a device for generating X-rays according to theinvention as shown in FIG. 1 comprises a metal housing 1 enclosing avacuum space 3, in which a source 5 or cathode for emitting electronsand a carrier 7 or anode provided with a material 9 which generatesX-rays as a result of the incidence of electrons are present. The source5, which is only schematically shown in FIG. 1, is mounted to thehousing 1 by means of a first mounting member 11 made from anelectrically insulating material. The carrier 7 is substantiallydisc-shaped, and the X-ray generating material 9, in this embodimenttungsten, is provided in the form of an annular layer on a main side 13of the carrier 7 facing the source 5. The carrier 7 is made from amaterial having a relatively high melting temperature, in thisembodiment molybdenum. Alternatively, the carrier 7 in its entirety maybe made from the X-ray generating material.

The carrier 7 is rotatable about an axis of rotation 15 which extendsperpendicularly to the main side 13. For this purpose, the devicecomprises a dynamic groove bearing 17, by means of which the carrier 7is journalled, and an electric motor 19, by means of which the carrier 7can be driven. The dynamic groove bearing 17 comprises an externalbearing member 21, which is mounted to the carrier 7, and an internalbearing member 23, which is mounted to the housing 1 by means of asupporting member 25 and a second mounting member 27. Between theexternal bearing member 21 and the internal bearing member 23, a bearinggap 29 is present which is filled with a liquid lubricant, in thisembodiment an alloy of gallium, indium, and tin. The motor 19, which isonly schematically shown in FIG. 1, comprises a rotor 31, which is alsopresent in the vacuum space 3 and is mounted to the external bearingmember 21, and a stator 33, which is present outside the vacuum space 3and is mounted to an external surface of the housing 1.

During operation, the source 5 generates an electron beam 35, whichpropagates via an electron beam path 37 extending perpendicularly to themain side 13 and which impinges upon the X-ray generating material 9 inan impingement position 39. X-rays 41 generated by the material 9 as aresult of the incidence of the electron beam 35 emanate from the vacuumspace 3 through a window 43, which is provided in the housing 1 andwhich is made from an X-ray transparent material, in this embodimentberyllium. Only a relatively small portion of the energy of the electronbeam 35 is converted into X-ray energy. A relatively large portion ofthe energy of the electron beam 35 is absorbed by the carrier 7, as aresult of which the carrier 7 is considerably heated during operation.Since, during operation, the carrier 7 is rotated about the axis ofrotation 15, the impingement position 39 follows a circular pathrelative to the carrier 7 over the annular layer of the X-ray generatingmaterial 9. As a result, the carrier 7 is uniformly heated in thecircumferential direction, so that excessive local temperatures of thecarrier 7 are avoided. Since the carrier 7 is present in the vacuumspace 3, transfer of heat from the carrier 7 to the surroundings of thedevice or to a cooling system of the device, necessary to avoidexcessive temperatures of the carrier 7, mainly takes place by heatconduction via the dynamic groove bearing 17 and the liquid lubricantpresent therein and by heat radiation from the surfaces of the carrier7.

A portion of the electrons of the electron beam 35 are scattered backfrom the impingement position 39, and accordingly a portion of theenergy of the electron beam 35 is converted into energy of the scatteredelectrons. The scattered electrons are caught for the greater part by aheat absorbing member 45, which substantially has the same electricalpotential as the carrier 7 and which is arranged in the vacuum space 3between the source 5 and the carrier 7, i.e. between the source 5 andthe impingement position 39. The heat absorbing member 45 issubstantially rotationally symmetrical relative to the electron beampath 37, and has a central opening 47 for the electron beam 35 and anelectron absorbing surface 49, which faces the carrier 7 and which willbe further discussed in detail hereinafter. The heat absorbing member 45is also used to absorb at least a portion of the radiant heat generatedby the carrier 7 when heated during operation. As a result of theabsorption of the scattered electrons and the radiant heat, the heatabsorbing member 45 is heated during operation. As shown in FIG. 2, theheat absorbing member 45 is in thermal connection with a cooling system51 of the device, which is only schematically shown in FIG. 2 andcomprises an annular sleeve 53, which is made from a material having arelatively high thermal conductivity, in this embodiment copper, and anannular heat exchanger 55, which is provided with a system of coolingchannels for a cooling liquid in direct thermal contact with the annularsleeve 53. The annular sleeve 53 and the heat exchanger 55 are arrangedconcentrically with respect to the electron beam path 37.

In view of the energy losses of the electron beam 35 as discussedbefore, a very high energy level of the electron beam 35 is necessary togenerate X-rays 41 of a sufficiently high energy level. In theembodiment shown in FIGS. 1 and 2, the source 5 is suitable to generatean electron beam 35 of approximately 200 kW. Experiments have shown thatapproximately 40% of the energy of the electron beam 35 is absorbed bythe heat absorbing member 45. If this amount of absorbed energy wasinstantaneously transferred from the heat absorbing member 45 to thecooling system 51, the necessary thermal capacity and dimensions of thecooling system 51 would be unacceptably high, or cooling systemproblems, such as boiling of the cooling liquid or melting ofthin-walled structures of the cooling system 51, would occur. In orderto avoid such substantial thermal capacities and dimensions of thecooling system 51 and to avoid such problems, the heat absorbingcapacity of the heat absorbing member 45 and the heat transferringcapacity of the thermal connection between the heat absorbing member 45and the cooling system 51 are such that, during operation, a rate ofheat absorption Q_(A) (in kW) by the heat absorbing member 45 issubstantially higher than a rate of heat transfer Q_(T) (in kW) via thethermal connection. As a result, the heat absorbing member 45 is used totemporarily store the heat absorbed by the heat absorbing member 45, andthe heat thus stored is gradually transferred from the heat absorbingmember 45 to the cooling system 51 during the time that the devicegenerates the X-rays 41 and afterwards when the device is not inoperation. Thus, in order to prevent excessive temperatures of the heatabsorbing member 45, the device has to be used discontinuously, i.e.after the generation of the X-rays 41 during a first period of time, thedevice should be out of operation for a second period of time, saidfirst and said second period of time depending on the energy level ofthe electron beam 35. In the embodiment shown, for example, the devicecan be used in a number of different modes of operation. In a first modeof operation, the electron beam 35 has an energy level of 200 kW duringa first period of time. After this, the device should be out ofoperation for a second period of time to allow the heated parts of thedevice to cool down again to a temperature close to the temperature ofthe cooling liquid. In a second mode of operation, the electron beam 35has an energy level of 100 kW during a period of time which isapproximately 3 times said first period of time, after which the deviceis out of operation to cool down again. In a third mode of operation theelectron beam 35 has an energy level of 60 kW during a period of timewhich is approximately 7 times said first period of time, after whichthe device is out of operation to cool down again. In a fourth mode ofoperation, the device continuously generates X-rays 41 at acomparatively low energy level of the electron beam 35.

In the device according to the invention, the intended relation betweenQ_(A) and Q_(T) as described before is achieved in that the thermalconnection between the heat absorbing member 45 and the cooling system51 comprises a thermal barrier φ which limits the rate ofcross-sectional heat transfer (in kW/K) occurring via the thermalconnection per unit of temperature difference between the heat absorbingmember 45 and the cooling system 51. It is noted that in the definitionof φ said temperature difference is the difference between an averagetemperature T_(A) of the heat absorbing member 45 and a temperatureoccurring at a thermal boundary between the thermal connection and thecooling system 51, i. e. at a location where the cooling liquid in thecooling system 51 is in direct thermal contact with the thermalconnection. In the first embodiment shown in FIGS. 1 and 2, said thermalbarrier comprises a mounting member 57 by means of which the heatabsorbing member 45 is mounted in the vacuum space 3 between the source5 and the carrier 7. The value of φ is effectively reduced in that adimension h_(B) of the mounting member 57, seen in a direction Xparallel to the electron beam path 37, is substantially smaller than adimension h_(A) of the heat absorbing member 45 in said direction X, sothat the mounting member 57 has a relatively small cross-sectional areaavailable for the conduction of heat. A predetermined limitation of thevalue of φ can be achieved by a suitable value of said cross-sectionalarea, i. e. by suitable value of h_(B). Since the value of φ, i.e. thecross-sectional thermal conductivity of the thermal connection betweenthe heat absorbing member 45 and the cooling system 51 is limited, arelatively high maximal temperature of the heat absorbing member 45 isallowed and achieved during the generation of the X-rays 41. As a resultof said allowed relatively high maximal temperature, only a relativelysmall mass and volume of the heat absorbing member 45 are required toprovide the heat absorbing member 45 with a sufficiently high heatabsorbing capacity. In the first embodiment, the heat absorbing member45 is made from molybdenum which has a relatively high meltingtemperature of approximately 2600° C. Alternatively, another materialhaving a relatively high melting temperature may be used, such astungsten or graphite. With such materials, relatively high temperaturesof approximately 2000° C. of the heat absorbing member 45 are allowed,so that a considerable reduction of the necessary mass and volume of theheat absorbing member 45 is achieved.

In the first embodiment shown in FIGS. 1 and 2, the value of φ isfurther reduced in that the mounting member 57 is made from a materialhaving a thermal conductivity which is smaller than a thermalconductivity of the material from which the heat absorbing member 45 ismade. In this embodiment the mounting member 57 is made from stainlesssteel, which is a very suitable material in view of its heat conductingproperties, its thermal expansion properties, and its mechanicalproperties. In the first embodiment, the value of φ is further reducedin that the mounting member 57 is in thermal contact with the heatabsorbing member 45 near a second side 59 of the heat absorbing member45 facing away from the carrier 7. Near this second side 59, duringoperation, the heat absorbing member 45 has a temperature which is lowerthan the average temperature T_(A) of the heat absorbing member 45 andlower than a temperature of the heat absorbing member 45 near a firstside 61 which faces the carrier 7, so that Q_(T) is further limited. Inthe first embodiment, as a result, Q_(T) has a maximal value ofapproximately 10 kW, which value occurs when the average temperate T_(A)is approximately 2000° C. Thus, the value of φ is approximately 5 W/K.In order to relate the value of φ to the total power and capacity of thedevice, a cross-sectional heat transfer coefficient θ (in K⁻¹) of thethermal connection between the heat absorbing member 45 and the coolingsystem 51 is defined as θ=φ/P_(max) wherein P_(max) (in kW) is a maximaloutput power of the source 5 allowed for continuous operation of thedevice. In the first embodiment P_(max) is approximately 25 kW, so thatθ is approximately 0.0002 K⁻¹. It is noted however that also for largervalues of θ a considerable reduction of the mass and volume of the heatabsorbing member 45 is already achieved. It has been found that a usefuland favorable reduction of the mass and volume of the heat absorbingmember 45 within the meaning of the invention is achieved for values ofθ smaller than approximately 0.0005 K⁻¹.

Since the maximal temperature of the heat absorbing member 45 is veryclose to the melting temperature of the material from which the heatabsorbing member 45 is made, local excessive temperatures in the heatabsorbing member 45 should be avoided. In the first embodiment shown inFIGS. 1 and 2, this is achieved as a result of the fact that the heatabsorbing member 45 is substantially rotationally symmetrical relativeto the electron beam path 37, and that the mounting member 57 is annularand concentric relative to the electron beam path 37. As a result, seenin a circumferential direction of the heat absorbing member 45, the heatabsorbing member 45 is uniformly warmed up by the electrons scatteredback from the impingement position 39, and the heat absorbed by the heatabsorbing member 45 is uniformly transferred from the heat absorbingmember 45 to the cooling system 51 via the mounting member 57.

The risk of local excessive temperatures, particularly near the electronabsorbing surface 49, is limited in that the electron absorbing surface49 has a concave shape as seen from the impingement position 39. It hasbeen found that the electrons scattered back from the impingementposition 39 have an energy level which depends on an angle α, as shownin FIG. 2, at which the electrons are scattered back relative to theelectron beam path 37. Said energy level is approximately proportionalto sin(2α), so that said energy level increases from approximately 0 atα=0° to a maximal value approximately at α=45°. As a result of the factthat the electron absorbing surface 49 is concave, a portion dS(α) ofthe electron absorbing surface 49, shown in FIG. 2 and available tocatch the electrons which are scattered back at a certain angle α, alsoincreases between α=0° and α=45°. By optimizing the shape of the concaveelectron absorbing surface 49, it is achieved that the energy absorbedper unit of area of the electron absorbing surface 49 is approximatelyconstant between α=0° and α=45°, so that at least near this portion ofthe heat absorbing surface 49 the risk of local excessive temperaturesis considerably reduced. For α>45°, the energy level of the scatteredelectrons decreases again, but the available portion of the heatabsorbing surface 49 increases further, so that local excessivetemperatures are not likely to occur near this portion of the heatabsorbing surface 49.

A further advantage of the device according to the first embodiment isthat the mounting member 57, which is necessary to mount the heatabsorbing member 45 in the vacuum space 3, also constitutes thenecessary thermal barrier in the thermal connection between the heatabsorbing member 45 and the cooling system 51. As a result, the deviceaccording to the first embodiment has a relatively simple constructionin that the number of parts of the device is limited. It is noted,however, that the invention also covers alternative embodiments in whichsaid thermal barrier constitutes an additional part of the device. Thesecond embodiment of a device according to the invention, which isschematically shown in FIG. 3, also has a relatively simple constructionin that the thermal barrier is a vacuum gap 63 which is present betweenthe heat absorbing member 45 and the cooling system 51. In FIG. 3, partsof the device according to the second embodiment which correspond withparts of the device according to the first embodiment, as shown in FIGS.1 and 2, are indicated by means of corresponding reference numbers. Inthe following, only the main differences between the devices accordingto the first and the second embodiment will be discussed.

The device according to the second embodiment mainly differs from thedevice according to the first embodiment in that the heat absorbingmember 45 of the second embodiment is mounted in the vacuum space 3 bymeans of two mounting members 65, 67 which are made from a thermallyinsulating material. The heat absorbing member 45 comprises a circularcylindrical outer wall, which is concentric with respect to the electronbeam path 37 and which constitutes a radiant heat transferring surface69 of the heat absorbing member 45. The annular sleeve 53 comprises acircular cylindrical inner wall, which is also concentric with respectto the electron beam path 37 and which constitutes a radiant heattransferring surface 71 of the cooling system 51. The vacuum gap 63 ispresent between said radiant heat transferring surfaces 69 and 71 and isannular and also concentric relative to the electron beam path 37. Inthis second embodiment, transfer of heat from the heat absorbing member45 to the cooling system 51 mainly takes place by radiation of heat fromthe radiant heat transferring surface 69 of the heat absorbing member 45via the vacuum gap 63 to the radiant heat transferring surface 71 of thecooling system 51, as a result of which the values of φ and θ for thethermal connection between the heat absorbing member 45 and the coolingsystem 51 are effectively reduced. Intended values of φ and θ areachieved in this second embodiment by suitable values of the surfaceareas of the radiant heat transferring surfaces 69 and 71 and by asuitable value of the width w of the vacuum gap 63.

1. A device for generating X-rays, which device comprises a source foremitting electrons, a carrier which is rotatable about an axis ofrotation and which is provided with a material which generates X-rays asa result of the incidence of electrons, a heat absorbing member arrangedbetween the source and the carrier, and a cooling system which is inthermal connection with the heat absorbing member, wherein duringoperation a rate of heat absorption by the heat absorbing member issubstantially larger than a rate of heat transfer via the thermalconnection, wherein the thermal connection between the heat absorbingmember and the cooling system comprises a thermal barrier which limitsthe rate of heat transfer, occurring via the thermal connection per unitof temperature difference between the heat absorbing member and thecooling system, in a predetermined manner; wherein the thermal barriercomprises a vacuum gap which is present between a radiant heattransferring surface of the heat absorbing member and a radiant heattransferring surface of the cooling system.
 2. A device as claimed inclaim 1, wherein the thermal barrier comprises a mounting member bymeans of which the heat absorbing member is mounted in the device, saidmounting member having a dimension, seen in a direction parallel to anelectron beam path of the source, which is substantially smaller than adimension of the heat absorbing member in said direction.
 3. A device asclaimed in claim 2, wherein the heat absorbing member is substantiallyrotationally symmetrical relative to the electron beam path, and themounting member is annular and concentric relative to the electron beampath.
 4. A device as claimed in claim 2, wherein the mounting member ismade from a material having a thermal conductivity which is lower than athermal conductivity of a material from which the heat absorbing memberis made.
 5. A device as claimed in claim 2, wherein the mounting memberis made from stainless steel.
 6. A device as claimed in claim 2, whereinthe heat absorbing member has a first side facing the carrier and asecond side facing away from the carrier, the mounting member being inthermal contact with the heat absorbing member near said second side. 7.A device as claimed in claim 1, wherein the heat absorbing member ismade from a material selected from the group consisting of molybdenum,tungsten, and graphite.
 8. A device as claimed in claim 1, wherein aside of the heat absorbing member facing the carrier has an electronabsorbing surface which is concave as seen from an impingement positionof the electrons on the carrier.
 9. A device for generating X-rayscomprising: a source for emitting electrons, a carrier which isrotatable about an axis of rotation and which is provided with amaterial which generates X-rays as a result of the incidence ofelectrons, a heat absorbing member arranged between the source and thecarrier such that substantially all of backscatter radiation strikes thesurface of the heat absorbing member, and a thermal connection locatedbetween the heat absorbing member and a cooling system, comprising aseparate and distinct component from said thermal connect and throughwhich cooling fluid flow, the thermal connection located such that thethermal connection is not exposed to the backscatter radiation, thethermal connection comprising a thermal barrier comprised of a differentmaterial than that which comprises cooling system such that the thermalbarrier limits the rate of heat transfer from the heat absorbing memberto the cooling system in a predetermined manner.
 10. The device asclaimed in claim 9, wherein the heat absorbing member is made from amaterial selected from the group consisting of molybdenum, tungsten, andgraphite.
 11. A device for generating X-rays comprising: a source foremitting electrons, a carrier which is rotatable about an axis ofrotation and which is provided with a material which generates X-rays asa result of the incidence of electrons, a heat absorbing member arrangedbetween the source and the carrier to absorb backscatter radiation, acooling system comprising a housing and a cooling fluid passingtherethrough; and a thermal connection located between, and distinctfrom, the heat absorbing member and the housing of the cooling system,the thermal connection comprising a thermal barrier that limits the rateof heat transfer from the heat absorbing member to the cooling systemhousing in a predetermined manner.
 12. A device as claimed in claim 11,wherein the heat absorbing member is made from a material selected fromthe group consisting of molybdenum, tungsten, and graphite.
 13. Thedevice as claimed in claim 11, wherein the thermal connection issubstantially free from exposure to backscatter radiation.
 14. Thedevice as claimed in claim 11, wherein the heat absorbing memberincludes a peripheral surface that contacts a surface of the thermalconnection, wherein the peripheral surface of the heat absorbing memberis substantially smaller than the contact surface of the thermalconnection.
 15. The device as claimed in claim 11, wherein the heatabsorbing member includes a peripheral surface, wherein a first portionof the peripheral surface contacts the thermal connection and secondportion of the peripheral surface does not contact any surfaces.
 16. Thedevice as claimed in claim 11, wherein the heat absorbing memberincludes a concave electron absorbing surface.